The present invention relates to a method for controlling rear wheels of a motor vehicle with a four-wheel steering system.
Various systems for steering front wheels and rear wheels have been proposed and developed for improving the stability of steering. Japanese Patent Publication 40-10728 discloses such a system.
The rear-wheel control systems may be divided into two types, one of which is a steering-angle-responsive type and the other is a steering-force-responsive type. In the former, the steering angle of the rear wheels is controlled in proportion to the steering angle of the front wheels. In the latter, the steering angle of the rear wheels is controlled in responce to the steering force for the front and rear wheels. The natural frequency with respect to the steering in the former is the same as a conventional two-wheel steering. On the other hand, the steering-force-responsive type has an advantage that the natural frequency may be set higher than that of the two-wheel steering. However, in the later, the damping of yawing with respect to the steering reduces compared with the two-wheel steering. Namely, the steering-force-responsive type has a disadvantage that there is a larger peak in frequency characteristic of the gain of yawing rate than that of the two-wheel steering
The steering force and the side force (cornering force) on a wheel is assumed to be proportional to the slip angles of the wheel. Thus, the steering-force-responsive type may be further divided into two types, one of which is a front-wheel steering force responsive type and the other is a rear-wheel steering force responsive type. In the former, the steering angle of the rear wheels is controlled in proportion to the slip angle of the front wheels and in the latter, the rear-wheel steering angle is controlled in proportion to the slip angle of the rear wheels. Since the latter controls the rear-wheel steering angle in accordance only with conditions of the rear wheel, the latter is superior to the former.
However, the latter also has the above-described disadvantage that a larger peak exists in the frequency characteristic of the gain than that of the two-wheel-steering.